The gas pipelines in use throughout the world transport billions of cubic feet of natural gas every day, at pressures in excess of 750 PSIG. As the gas is forced through these pipelines, friction occurs. Friction results in pressure loss, which results in a loss of capacity and lost sales opportunities. To minimize the loss due to frictional pressure drop, traditional pipeline companies install large booster compressor stations.
The natural gas transmission systems were originally designed with thousands of miles of pipe, utilizing compressor stations at 80 mile intervals to boost the gas pressure. As system capacity requirements increased, intermediate stations were installed, shortening the distance between stations to 40 miles. The distance between stations represents the industry's attempt to balance compressor station costs, pipeline costs, and available capacity. With costs exceeding $40 million for a typical station, most companies could not economically justify locating their stations closer together.
Compressor stations boost a pipeline's capacity by increasing the pressure. Increasing pressure increases the gas density, which allows the same quantity of energy to occupy less space. Maintaining high pressure helps in two ways: higher pressures create denser gas that requires less space and flows at a lower velocity, thus introducing less friction in the pipeline and lessening the pressure drop for equivalent gas energy packets; and denser gas allows more gas to be packed into the pipeline.
Packing is very important to pipelines. If a pipeline operator knows that the demand in the morning is going to be higher than what the company can normally deliver, the operator can "pack" the line the night before, storing extra gas in the line. This pack allows the customer to draw down the pressure. Therefore, it is advantageous to keep the pressure as high as possible for as long as possible.
Due to the pressure losses that occur along the length of a natural gas pipeline, compressor stations are needed at various intervals to maintain the pressure and flow of natural gas through the pipeline. Typical compressor stations use gas power engines to drive the compressors. These compressor stations suffer from numerous disadvantages. For example, compressors are not efficient when partially loaded and tend to generate significant amounts of noise in the surrounding environment. Additionally, conventional compressors are very sensitive to stops and starts. Therefore, numerous starts and stops can significantly reduce the useful life of the unit.
Moreover, conventional compressors are difficult to replace in the event of a failure. During an outage, pipeline companies tend to repair engines in place, causing down time for the pipeline. Furthermore, conventional compressors are rarely interchangeable. If the pipeline operator chooses to change equipment or modify the plant's piping, the operator is usually limited to the unique footprint of that equipment. Thus, unless the operator is willing to scrap the existing equipment, it would be impractical to modify or change conventional compressors. In addition, emissions from gas-driven compressors continue to be an increasingly greater problem when trying to comply with increasingly strenuous environmental regulations.
FIG. 5 is an example of a conventional compressor station. The compressor station comprises underground gas pipeline 100 through which gas flows from left to right as indicated by the arrow. Blocking valve 102 is provided to prevent the gas from flowing in a circular pattern. and is normally closed when the station is operating. On the upstream side of pipeline 100 is a suction header 104. The suction header 104 contains various piping components that are conventional in the art such as station block valve 105 and scrubber 107. The suction header 104 supplies gas to the centrifugal compressor 116 which is driven by motor 114. In order to duct the gas to the compressor 116, there is provided a 45.degree. elbow 106 which feeds a rolled 90.degree. elbow 108. On the output side of compressor 116 similar piping is provided. The output flows through rolled 90.degree. elbow 110 and then through 45.degree. elbow 112 in the discharge header 118 and finally reenters the pipeline 100. One problem with the conventional compressor station shown in FIG. 5 is that there is a pressure loss due to the friction of the gas in the pipelines going through bends, such as the 90 and 45.degree. elbows. Therefore, it is desirable to eliminate these elbows in a compressor station. Also, since the gas enters the compressor 116 at a right angle to the shaft of motor 114 there is a chance that piping forces could be imparted on the intake or output nozzles of the compressor 116. This can cause casing distortion to the compressor 116 and damage the compressor and motor 114. It can also cause misalignment of the motor 114 and the compressor 116 which causes vibration in the compressor station. This vibration can, in turn, cause the compressor to shut down and wear out mechanical components prematurely. Further, this misalignment makes it difficult to adequately seal the compressor 116. Because of maintenance issues, conventional compressor stations are typically constructed in buildings above ground which require additional space and costs for the station. The building also must be insulated to reduce noise. The same problems occur in a variety of gas compression systems and are not limited to natural gas compressor stations.
Though the previous examples represent some of the major deficiencies of conventional compressor stations, this list is by no means exhaustive.
The present invention overcomes these deficiencies and provides further improvements and advantages which will become apparent in view of the following disclosure.